Elected consul for life, he assumed the hereditary title of emperor in 1804. His administrative, military, educational, and legal reforms (notably the Code Napoléon) made a lasting impact on French society. War with England was renewed, and extended to Russia and Austria. Forced by England's naval supremacy at Trafalgar (1805) to abandon the notion of invasion, he attacked the Austrians and Russians, gaining victories at Ulm and Austerlitz (1805). Prussia was defeated at Jena and Auerstadt (1806), and Russia at Friedland (1807). After the Peace of Tilsit, he became the arbiter of Europe. He then tried to cripple England with the Continental System, ordering the European states under his control to boycott British goods. He sent armies into Portugal and Spain, which resulted in the bitter and ultimately unsuccessful Peninsular War (1808–14).

In 1809, wanting an heir, he divorced Joséphine, who was childless by him, and married the Archduchess Marie Louise of Austria, a son being born in 1811. Believing that Russia was planning an alliance with England, he invaded (1812), defeating the Russians at Borodino, before entering Moscow, but he was forced to retreat, his army broken by hunger and the Russian winter. In 1813 his victories over the allied armies continued at Lützen, Bautzen, and Dresden, but he was routed at Leipzig, and France was invaded. Forced to abdicate, he was given the sovereignty of Elba (1814). The unpopularity which followed the return of the Bourbons motivated him to return to France in 1815. He regained power for a period known as the Hundred Days, but was defeated by the combination of Wellington's and Blücher's forces at Waterloo. He fled to Paris, abdicated, surrendered to the British, and was banished to St Helena, where he died. In all the areas of Europe under Napoleonic rule, the Code Napoléon became law. A great number of political and social reforms resulted, including the abolition of feudalism and serfdom, freedom of religion in most occupied states, the granting of constitutions, universal male suffrage, and parliaments. Administrations, judiciary systems, and a free education system based on the French model were set up. Higher education was made available to all qualified applicants regardless of religion or social class.